One of the benefits of photosynthetic micro-organisms such as microalgae and cyanobacteria lies in the original composition thereof.
However, the majority of these compounds involve the application of stress conditions to the culture so as to make use of the significant metabolic flexibility of this type of micro-organism to achieve the biosynthesis of the desired compound.
If this compound is intracellular (such as pigments in particular), it is necessary to implement a second extraction step which, in the majority of cases, causes irreversible damage to the culture (grinding, thermal or osmotic shock, cellular disintegration, etc.).
Therefore, industrial productions based on this principle use discontinuous productions, successively alternating biomass production, stress condition application phases, followed by harvesting, extraction and purification phases.
The main drawback of discontinuous productions is associated with the low growth rate of photosynthetic micro-organisms (compared to heterotrophic micro-organisms such as bacteria or yeasts), preventing frequent harvesting, or, at the very least, involving work with a plurality of production systems in parallel.
For some compounds, it is possible to implement an original extraction technique, wherein only the compound is extracted, without altering the cell significantly (biocompatible extraction). This enables the continuous production of the compound, while preventing the repetition of the growth and stress phase.
Indeed, if this technique is associated with continuous biomass production in a photobioreactor, once the initial growth phase has been carried out, the stress conditions subsequently applied can be theoretically maintained indefinitely, provided that the compound produced is continually extracted.
This results in a non-negligible gain in production compared to discontinuous methods, avoiding the losses associated with the latent period before obtaining a further biomass having the desired cellular composition.
Such a continuous production-extraction technique was proposed by Hejazi and Wijffels in 2003 (patent document published under the number EP-1 501 937) and is described with reference to FIG. 1.
This device is also based on the research detecting the existence of biocompatible solvents enabling the extraction of reusable compounds, in this instance β-carotene from the microalga Dunaliella salina (research by Leon et al., 2003).
As illustrated in FIG. 1, this device comprises a photobioreactor wherein the culture 1 containing the compound to be extracted and the solvent 2 wherein the compound is progressively extracted coexist (biphase system).
Although it is operational, this method displays various limitations. Indeed, the extraction rate remains relatively low, due to the difficulty placing the two non-miscible phases (cultures in aqueous medium and hydrophobic solvent) in contact. As the contact interface remains small, there is a significant resulting limitation in material transfer. The solution consists of mixing the whole very vigorously, which, however, rapidly reaches a limit determined by the fragility of the cultured cells.
A further drawback lies in the impossibility to impose optimal conditions, as both processes (biosynthesis and extraction) are different. For example, as extracted β-carotene is photosensitive, the use of strong lighting (stress condition required for biosynthesis) for culture makes it necessary to draw off the solvent charged with pigment regularly before degradation.